This invention relates to a digital data processing system, and more specifically, to a multiprocessor system comprised of a plurality of digital data processors sharing common control circuitry, memory and peripherals.
A number of digital data processing systems have been built heretofore based on multiple processors. These systems, referred to as multiprocessor systems, have had many diverse constructions. They have ranged from independent processors with no interconnection, through tightly coupled processor networks that communicate with each other by passing messages, to multiprocessors that share common control circuitry, memory and peripherals. An excellent discussion of multiprocessor systems and of the trade-offs involved in system designs depending upon the degree of coupling and sharing involved in the systems is presented in Chapter 16 of the book entitled Computer Engineering, by C. Gordon Bell et al, Digital Press (1978).
Until recently, use of multiprocessor systems have been restricted to highly specialized applications, such as those requiring high availability, or high reliability and performance beyond that capable with a single processor with a given technology. One reason for this has been the widespread acceptance of the philosophy that a better higher performance single processor system can always be built. High performance in single processor systems, however, is generally achieved only at the expense of considerable additional design and engineering development which often restricts availability and involves trade-offs in system reliability and flexibility. Further, for the end user desiring higher performance, the substitution of new single processor system for an existing system can involve problems of system adaptation and training.
The advent of low-cost microprocessors with substantial functionability, such as those of the PDP-11 data processing family manufactured by the assignee hereof, has created new possibilities for multiprocessor systems. Because of the high availability and low cost per compute cycle of such microprocessors, multiprocessor systems comprised thereof have the potential of providing computational capabilities that span ranges typically covered by many conventional single processor systems at comparable costs. For the end user, they offer the advantage of enabling modular incremental growth over limited and well-defined performance ranges with minimum training and other field service costs.